Cassegrain antenna for scanning with elliptically shaped beam



L. SCHWARTZ CASSEGRAIN ANTENNA FOR SCAN ELLIPTI Fi 3,392,397 NING WITHCALLY SHAPED BEAM led Feb. 15, 1966 July 9, 1968 I NVENTOR.

LEONARD SCHWARTZ BY ATTORNEY United States Patent 3 392,397 CASSEGRAINANTENNA FOR SCANNING WITH ELLIPTICALLY SHAPED BEAM Leonard Schwartz,Yonkers, N.Y., assignor to General Precision Systems Inc, a corporationof Delaware Filed Feb. 15, 1966, Ser. No. 527,549 6 Claims. (Cl.343-461) The present invention relates to antennas for directionallytransmitting radio frequency (RF) energy in a particular shape beam.

One aspect of the present invention relates to a novel antennaarrangement for propagating an elliptically shaped beam of RF energy.

Another aspect relates to a novel compact antenna arrangement forindependently propagating each of three separate beams by a composite,compact antenna.

A further aspect relates to a novel antenna arrangement for propagatingan elliptical beam and rotating the elliptical beam while maintainingthe orientation of the elliptical shape.

Generally, the novel antenna system described herein directionallytransmits an elliptical shaped beam which is conically scanned, thusrotating the elliptical beam in a circle. This beam is referred to asthe main beam. The present state of the art is such that side lobes ofRF adjacent to the transmitted beam are not eliminated by antennadesign. In order to eliminate undesirable effect of the side lobes theantenna system includes the generation of two elliptical auxiliary beamspositioned respectively at the sides of the main beam which mayessentially blanket the side lobes of the main beam.

The novel antenna system is a composite antenna of at least twodifferent types combined in compact arrangement. One of the antennas maybe referred to as a Cassegrainian type antenna while another of theantennas of the composite system may be referred to as a hornreflectorarrangement.

The main beam is propagated by the Cassegrainian type antenna whichemploys the novel combination of a dielectric rod feed, which is taperedalong the major part of its length. The feed appears through the vertixof a modified parabolic reflector or main dish and directs RF energy toan eccentrically rotated hyperboloidal reflector or subdish. The feed isparticularly located at one focal point of the hyperbolic reflector andthe hyperbolic reflector images the real source of RF into a virtualsource at its other focal point, the other focal point being coincidentwith the focal point of the parabolic reflector.

The elliptical shape of the propagated beam may be provided in any oneof several ways: feed modification; subdish modification or main dishmodification, the latter being the preferred method and the arrangementwith which the present application is concerned. An elliptically shapedbeam may be provided by providing an unmodified feed and subdisharrangement and providing a main dish having a substantially ellipticalconfiguration, from a plan view. The desired structure of the main dishmay be provided by modifying a parabolic reflector by reducing theaperture dimension of the reflector in the azimuth plane. The resultingpattern will have a beam width equal to:

YA/D' where K is a constant determined by the illumination taper at theedge of the aperture; 7\ is the wave length of the RF energy and D isthe length of the minor axis.

The main antenna of the complex, compact antenna system provides ascanned beam from a feed which produces a circular radiation patternwhich illuminates an eccentrically driven hyperboloidal reflector whichre- 3,392,397 Patented July 9, 1968 radiates the power into a generallyelliptically shaped (modified) parabolic reflector. The beam is scannedby rotating the hyperboloid or subdish so that its center makes a circleabout the focus of the Paraboloid. The amount of eccentricity (radius ofthe circle) determines the squint angle of the emerging beam.

One limitation of the Cassegrainian type antenna is the blocking effectsof the hyperboloidal subdish and feed. These blocking effects reduce theantenna gain, widen the beam width and increase the side lobe level. Thecondition for minimum aperture blocking occurs When the front blockingof the subdish is made substantially equal to the back blocking of thefeed. The minimum blocking diameter is given by the expression:

where D min. is the diameter of the subdish; k is the ratio of effectivefeed aperture diameter to its blocking diameter; F is the focal lengthof the parabolic reflector and A is the operating wave length of the RFenergy.

It becomes clear from the Equation 2 that to minimize the blockingdiameter it is necessary to use as short a focal length as possible. Inthe successful practice of the invention an eighteen inch (18") diameterparabolic reflector was used in which the focal length was 4.5 inches.Equation 2 also indicates that an optimum feed for the Cassegrainianantenna is one in which the effective feed area is equal to the blockingarea, that is, where k=1. This condition exists for a dielectric rodantenna for which the effective receiving area is greater than itsphysical blocking area. This is due to the fact that the end-firedielectric rod antenna achieves its gain through length rather thancross-section.

Thus, solving Equation 2 where F=4.5, k=1 and )\=.76 the D min.=2.6.Therefore, for example, an 18" parabolic reflector having a 4.5" focallength may use a subdish of 2.6" in diameter.

The diameter of the subdish being established, its shape remains to bedetermined. The equation for the equivalent focal length (F of aCassegrainian antenna tan 0 tan are) 3- where F is the focal length ofthe parabolic reflector; 0., is half angle subtended by the parabolafrom its focal point and 0, is half angle subtended by the hyperboloidfrom its feed point.

Equation 3 shows that as 0 decreases, large effective focal lengths arepossible. However, as 0, decreases the gain of the feed required tocorrectly illuminate the subdish =increases. Since part of the energyilluminating the dish is spilled over, it is desirable to keep the gainof the feed low. Assuming the spillover gain is equal to theillumination edge taper times the gain of the feedhorn with the feedgain at 10 db and the taper at 10 db the peak gain of the spillover is 0db. With an antenna gain of 34 db, for example, the spillover power willnot be a problem for a 10 db feed.

An example of a compromise between F (the focal length of theCassegrainian antenna) and the spillover power may be:

F... 2 4 thus, the off axis scanning characteristics will be determinedby the equivalent:

D whlere D is the major diameter of the modified main dis (X and Y areunknowns) where I b=a /e 1 (10) and for the exemplary parameters chosen,the values of the constant are:

Employing the presented equations a Cassegrainian type antenna may beprovided for propagating a conically scanned elliptical beam of energy.

It is therefore an object of the present invention to provide a novelantenna arrangement for popagating an elliptically shaped beam of RFenergy.

Another object is to provide an antenna arrangement for propagating anelliptical beam of RF energy and for rotating the elliptical beam whilemaintaining the same orientation of the elliptical shape.

Another object is to provide a composite compact antenna system fortransmitting a conically scanned elliptical beam with two auxiliaryelliptically shaped beams also transmitted by the same system forcovering adjacent side lobes on either side of the main beam.

These and other objects will become more apparent from reading thefollowing detailed description of an embodiment of the invention inwhich:

FIG. 1 is a plan view of the main dish with details of the auxiliaryantenna feeds omitted for the sake of clarity, and

FIG. 2 is a cross-section view, from the side of the composite antennasystem for propagating a conically scanned elliptical beam and twoauxiliary elliptical beams.

Referring more particularly to FIG. 1, the surface contour of the maindish or main reflector is illustrated with the major diameter D andminor diameter D indicating the major and minor diameters of thereflector, respectively. The circle 12 has its center at the axis of thediameters and illustrates the path of the center of the subdish 10 inits rotational movement. The arrows 11 represent the eccentric movementor rotation of the subdish 10.

FIG. 1 illustrates one method of forming the elliptical structure of thereflector D/D' from a round parabolic reflector, such as represented by14. The cross-hatched areas of 15a and 1512 may be covered with amicrowave absorbing material.

As shown above, the equations provided may be used for mathematicallydetermining the minor diameter D' and the contour of the sides of theelliptically shaped reflection area. The length and width measurementsof the beam will be determined by the size of the reflector and thesurface or plan and side or profile contours of the reflector. By way ofexample, a beam having an elevation width of 3.20 degrees and an azimuthwidth of 4.7 degrees "4 has been provided by the arrangement shown Wherea parabolic reflector, such as 14, had a major diameter D measuring 18inches with the sides of the elliptically shaped reflector extending toa minor diameter D measuring 8 inches.

It will be appreciated that-the drawing FIG. 1 has omitted apparatus forpropagating the auxiliary or cover beams. This has been omitted for thesake of clarity. In addition, the motor employed for rotating thesubdish 10 and a network for suspending the motor and subdish have alsobeen omitted. The dielectric rod feed is hidden by the subdish 10. Theomitted components may be seen in the profile view in FIG. 2.

In order to provide a composite antenna system for propagating a mainbeam and two auxiliary or cover beams, the reflectors for each of theauxiliary beams may be positioned in the unused areas 15a and 15b of theparabolic reflector 14. The feed may be in the form of a wave guidehorn, one horn for each reflector, positioned to illuminate theparticular reflector for providing the auxiliary beams.

This may be seen in FIG. 2 which illustrates a profile view along lineAA of a composite, compact antenna system positioned within a parabolicreflector, part of which is used as part of the antenna for transmittingthe main beam.

The sections 15a and 15b in FIG. 2 correspond to the identically labeledsections in FIG. 1. The minor diameter D is shown in profile location onthe parabolic reflector 14 and corresponds to the minor diameter D' inFIG. 1.

In utilizing the areas 15a and 15b for positioning reflectors for theauxiliary beams, sectors of another parabolic reflector may bepositioned above the areas 15a and 15b such as shown by and 15d,respectively. The other parabolic reflector, from which the sectors,represented by 150 and 15d, may be cut, would be a reflectorsubstantially physically similar to reflector 14.

The sectors may be inserted over the unused reflection areas with eachsector 15c and 15d being tilted, for example, some 7. Radio frequencyenergy is fed by open wave guide horns 20 and 21, respectively, eachpositioned above the reflector sector with which it is associated andeach tilted some 30", for example, in opposite directions.

Since each of the auxiliary beams, propagated by the auxiliary antennas,are intended to be stable beams, as opposed to the scanned or rotatedmain beam, the sector forming the reflector (150 or 15d) need not beexactly physically symmetrical, in plan form, as is desired of the maindish of the main antenna. Thus the sectors 15c and 15d may besubstantially corresponding to the shape of the areas 15a and 1512respectively.

In order to provide a wide azimuth pattern of auxiliary or cover beam,each of the horns 20 and 21 may be in the form of a dual horn. In thepreferred form, each of the horns 20 and 21 is in the form of apyramidal horn with an E plane bifurcation. This may be formed by aseptum 22 which serves to split the power propagating into the horn intotwo equal parts so that at the exit aperture there are essentiallytwo-in-phase horns sideby-side. This construction positions the phasecenter of each horn of the dual horn at different offset distances fromthe focal point of the parabola sector so that the peak of each beampropagated by the dual horn arrangement will occur at a differentazimuth angle producing a wider pattern than obtainable from a singlehorn of the same combined aperture.

Each of the horns, 20 and 21, also includes its respective wave guideline for conducting microwave energy from a source (not shown) to thehorn.

Also shown in FIG. 2 is a representation of a motor 16 for driving thesubdish 10. A shaft, 17, couples the motor 16 to the subdish 10 and, aswill be readily seen, such coupling is at an off-center position of thesubdish. Thus the subdish is rotated eccentrically about the axis of thediameters D and D' as indicated in FIG. 1.

The dielectric rod feed 18 is also shown, positioned at the centerposition along diameters D and D.

The composite, complex antenna assembly may also include a radome, notshown, for protecting the apparatus from the elements.

It will be obvious that the motor 16 and subdish must be suspended insome manner so as to hold the motor and subdish in position. This may beaccomplished by a cross network of rods which are sufficiently small indiameter so as not to undesirably interfere with energy radiated fromthe antenna assembly, but sufliciently strong to support the motor andsubdish and hold such components in place.

A preferred network may consist of at least four support rods (two ofwhich 24 and 25 are shown) supporting a collar or frame 26 which maysupport the motor 16. The subdish 10 would then be supported by theshaft 17 which serves also to rotate the subdish.

It thus becomes obvious that an antenna for transmitting an ellipticalbeam (an unscanned beam) may be made by slightly modifying thearrangement shown.

If, for example, the hyperboloid reflector 10 were positioned so thatits center were in coincidence with the axis or crossing point of thediameter -D and D of the parabolic reflector 14 and the means forrotating the hyperboloid reflector 10 were eliminated then an antennafor propagating a highly directional elliptically shaped beam isprovided.

Considering this latter arrangement two ways have been shown forproviding an elliptically shaped beam.

Thus there has been described and shown the preferred arrangement forpracticing the invention. Obviously, other arrangements and structureemploying the same principles disclosed herein may be used, as will befamiliar to those skilled in the art, without departing from the spiritof the invention as defined in the appended claims.

What is claimed is:

1. A microwave energy antenna for directionally transmitting a conicallyscanned elliptically shaped beam of microwave energy including;

a modified parabolic reflector having the aperture dimension of saidreflector reduced in the azimuth plane for providing a beam width equalto where K is a constant determined by the illumination taper at theedge of the aperture, A is the wave length of said microwave energy andD is the length of the minor axis,

a hyperboloidal reflector positioned above said modified reflector andat the focal point thereof for radiating energy to saidmodifiedreflector.

a dielectric rod feed positioned on the axis of said modified reflectorfor radiating microwave energy to said hyperboloidal reflector, and

means for revolving the hyperboloidal reflector eccentrically about theaxis of said modified parabolic reflector.

2. A microwave energy antenna as in claim 1 and in which said dielectricrod feed is tapered along its length for disturbing microwave energyover the radiating surface of said hyperboloidal reflector.

3. A microwave energy antenna system for directionally transmitting aconically scanned elliptically shaped beam of microwave energy and atleast two auxiliary beams azimuthly positioned for covering side lobesof the conically scanned elliptically shaped beam including;

a modified parabolic reflector having the aperture di- 6 mension thereofreduced in the azimuth plane for providing a beam width equal towhere Kis a constant determined by the illumination taper at the edge of theaperture, A is the wave length of the microwave energy and D is thelength of the minor axis,

a hyperboloidal reflector positioned at the focal point of the saidmodified reflector for radiating microwave energy to said modifiedreflector,

a dielectric rod positioned on the axis of said modified reflector forradiating microwave energy to the radiating surface of saidhyperboloidal reflector,

means for revolving said hyperboloidal reflector so that the center ofthe radiating surface of said hyperboloidal reflector revolves about theaxis of said modified parabolic reflector,

first and second auxiliary reflectors each having substantially the sameconfiguration as said modified reflector, the minor axis of said firstand second auxiliary reflector and said modified reflector beingsubstantially parallel and aligned,

means including a wave guide horn for radiating microwave energy on tothe reflecting surface of said first auxiliary reflector for propagatinga beam of microwave energy in an elliptically shaped configuration andpositioned adjacent one side of the said scanned beam, and

means including a second wave guide horn for radiating microwave energyon the reflecting surface of said second auxiliary reflector forpropagating a second beam of microwave energy in an elliptically shapedconfiguration and positioned adjacent the other side of the said scannedbeam.

4. A microwave energy antenna system as in claim 3 and in which saidwave guide horn and said second wave guide horn are each in the form of,

a pyramidal horn with an E plane bifurcation for dividing each said horninto two in-phase horns.

5. A microwave energy antenna system as in claim 3 and in which said.modified parabolic reflector is formed by covering part of a roundparabolic reflector with microwave absorbent material so that only anelliptically shaped portion of said round parabolic reflector serves toreflect energy radiated from said hyperboloidal reflector.

'6. A microwave energy antenna system as in claim 3 and in which saidmodified parabolic reflector is formed by covering part of a roundparabolic reflector with microwave energy absorbent material so that anelliptically shaped portion of said round parabolic reflector centeredabout the axis of said round parabolic reflector serves to reflectenergy radiated from said hyperboloidal reflector and in which saidfirst auxiliary reflector is supported by a portion of said roundparabolic reflector covered by microwave energy absorbent material andsaid second auxiliary reflector is supported by another portion of saidround parabolic reflector covered by microwave energy absorbent materialso that the three antennas are essentially contained within the confinesof a single round parabolic reflector.

References Cited UNITED STATES PATENTS 2,531,454 11/1950 Marshall343-761 ELI LIEBERMAN, Primary Examiner.

1. A MICROWAVE ENERGY ANTENNA FOR DIRECTIONALLY TRANSMITTING A CONICALLYSCANNED ELLIPTICALLY SHAPED BEAM OF MICROWAVE ENERGY INCLUDING; AMODIFIED PARABOLIC REFLECTOR HAVING THE APERTURE DIMENSION OF SAIDREFLECTOR REDUCED IN THE AZIMUTH PLANE FOR PROVIDING A BEAM WIDTH EQUALTO